Drum contact freezers are widely used for rapid freezing of food products and other materials. Such devices freeze a material by placing the material on an outer contact surface of a drum while the drum is rotated. Refrigerant such as FREON, ammonia, or D-limonene-carbon dioxide is provided to the interior of the drum to cool the external surface thereof and normally operate in a range of -100.degree. F. to -30.degree. F. The material in contact with the outer drum surface is frozen by heat transfer. Additionally, some drum freezer systems enhance freezing of the material by providing an additional coolant exterior of the drum near the material to be frozen. For example, the additional coolant may be provided on the outer drum surface or on a web which is used to press the material to be frozen against the outer drum surface.
As the drum freezer and the material to be frozen rotate together, the refrigerant within the drum sequentially contacts different portions of the inner drum surface (unless the drum is completely filled). A recurring problem with known contact drum freezer systems has been build up of a stagnant layer on the internal surface of the drum due to the manner in which the refrigerant circulates and contacts different portions of the inner drum surface. This stagnant layer on the inner drum surface can cause a significant reduction in the heat transfer rate.
When using D-limonene-carbon dioxide and when moisture is present the stagnant layer tends to create an insulating layer thereon and effectively increases the thickness of the drum through which heat must be transferred. Thus the rate at which food may be frozen is decreased and in some cases causes inadequate freezing thereof and resultant health concerns. The overall efficiency of the known systems is therefore less than desirable and is further decreased by the need for relatively frequent shutdowns to clean the stagnant layer from the drum surfaces.
Due to limitations on efficiency and the rate of heat transfer, known freezer systems also have a fairly narrow range of food products that can be frozen. Some food products such as liquid eggs (and other liquids) are inadequately frozen and/or have a resulting bad taste when using the freezers known in the art.
Therefore, there is a need for a drum contact freezer system in which the stagnant layer on the interior surface of the drum may be reduced or eliminated to enhance the rate of heat transfer and allow faster freezing plus a wider range of allowable food products.